I had low oil pressure at idle when warm. Checked oil pan (pick up tubes etc.), changed oil and filter and eventually replaced the pump. Seemed to solve the issue for a day, but now it is right back to same issue. Low oil pressure at idle when warm. It is also possibly idling low. Not sure how accurate the tach is, but it will dip below 500 occasionally. No check engine light, just the oil pressure light.

 

How many miles? Could be bearing wear.

 

Psi test to eliminate oil switch

 

This. I've replaced many, many oil pump gearsets and have had two or three that had little to no oil pressure to begin with and had little to no oil pressure after the gear swap. Subsequent disassembly showed significant rod bearing wear.

But, the switch is a cheap test.

 

The tach is no where near accurate enough to read rpm, you need to use a OBDII live data scanner to see the actual RPM.
If the rpm are to low the oil light will come on because the oil pump is pumping to slow.

 

The switch was already replaced. Its a completely rebuilt motor with about 15k on it.

 

What oil did you use?
Have you checked the rpm yet with a live data scanner?

 

Outside of the engine, does your D2 have an engine oil cooler in addition to the trasmission cooler they all have? An oil cooler or line to it can also be partially obstructed.

 

With low miles on the rebuild I would think bearing wear is not the issue. Maybe the o ring on oil pickup tube is bad and it was missed at rebuild?

 

Good point, could be o-ring making it suck air and not enough oil at higher speed as well, could be bolts on pickup loose (great place for locktite), could be trash on pickup screen. See 30 and 31 on attached drawing.

Spike is also right, if idle way below 700 rpm, the displacement pump output will be low because of physics, not an oil leak. You may wish to repeat oil PSI test using a scanner for tachometer readings. Would expect above 10 PSI warm at 700 rpm. (spec with auto tranny is 760 rpm +/- 50 rpm)